The present invention relates to a device which allows automatic focusing of an automatic focusing camera even when various conversion lenses are used.
Automatic focusing cameras have been recently commercially available which automatically detect a distance between the subject and the camera to focus the photographic lens on the subject. There are two main automatic focusing methods: a method in which light from a subject is received to measure the distance between the subject and the camera; and a method in which light is emitted by a camera to the subject, and the reflected light from the subject is received to measure the distance between the subject and the camera. In both these methods, a distance measuring mechanism is interlinked with a photographic moving mechanism to automatically focus the photographic lens on the subject, since a focal length of a lens mounted on an automatic focusing camera (to be referred to as a photographic lens hereinafter) is set to be constant.
However, such a device cannot be used when a conversion lens such as a telephoto lens or a wide-angle lens is attached to the camera. This is because these conversion lenses have different focal lengths from that of the standard lens.
Therefore, the focal length of the photographic lens may not be kept constant. When a conversion lens is attached to the photographic lens, a synthetic or effective focal length results which is the product of the focal length of the standard lens and the magnification of the conversion lens. In this case, the required range of movement of the photographic lens to provide focusing at a given distance between the subject and the camera is altered.